Improve Parametric Scopes documentation

guides/source/routing.md

@@ -511,7 +511,7 @@ When using `magazine_ad_path`, you can pass in instances of `Magazine` and `Ad`
 <%= link_to 'Ad details', magazine_ad_path(@magazine, @ad) %>
 ```
 
-You can also use `url_for` with a set of objects, and Rails will automatically determine which route you want:
+You can also use [`url_for`][ActionView::RoutingUrlFor#url_for] with a set of objects, and Rails will automatically determine which route you want:
 
 ```erb
 <%= link_to 'Ad details', url_for([@magazine, @ad]) %>
@@ -537,6 +537,8 @@ For other actions, you just need to insert the action name as the first element
 
 This allows you to treat instances of your models as URLs, and is a key advantage to using the resourceful style.
 
+[ActionView::RoutingUrlFor#url_for]: https://api.rubyonrails.org/classes/ActionView/RoutingUrlFor.html#method-i-url_for
+
 ### Adding More RESTful Actions
 
 You are not limited to the seven routes that RESTful routing creates by default. If you like, you may add additional routes that apply to the collection or individual members of the collection.
@@ -1164,15 +1166,29 @@ resources to use `photos_path` and `accounts_path`.
 
 NOTE: The `namespace` scope will automatically add `:as` as well as `:module` and `:path` prefixes.
 
-You can prefix routes with a named parameter also:
+#### Parametric Scopes
+
+You can prefix routes with a named parameter:
 
 ```ruby
-scope ':username' do
+scope ':account_id', as: 'account', constraints: { account_id: /\d+/ } do
   resources :articles
 end
 ```
 
-This will provide you with URLs such as `/bob/articles/1` and will allow you to reference the `username` part of the path as `params[:username]` in controllers, helpers, and views.
+This will provide you with paths such as `/1/articles/9` and will allow you to reference the `account_id` part of the path as `params[:account_id]` in controllers, helpers, and views.
+
+It will also generate path and URL helpers prefixed with `account_`, into which you can pass your objects as expected:
+
+```ruby
+account_article_path(@account, @article) # => /1/article/9
+url_for([@account, @article])            # => /1/article/9
+form_with(model: [@account, @article])   # => <form action="/1/article/9" ...>
+```
+
+We are [using a constraint](#segment-constraints) to limit the scope to only match ID-like strings. You can change the constraint to suit your needs, or omit it entirely. The `:as` option is also not strictly required, but without it, Rails will raise an error when evaluating `url_for([@account, @article])` or other helpers that rely on `url_for`, such as [`form_with`][].
+
+[`form_with`]: https://api.rubyonrails.org/classes/ActionView/Helpers/FormHelper.html#method-i-form_with
 
 ### Restricting the Routes Created